1. Field of the Invention
This invention relates to ferritic stainless steel sheets suitable for containers and piping elements for organic fuels such as gasoline, methanol and the like. In particular, the invention relates to a ferritic stainless steel sheet which can be readily shaped into fuel tanks and fuel pipes and which is resistant to organic fuels, particularly deteriorated gasoline containing organic acids produced in the ambient environment. The invention also relates to a method for making the ferritic stainless steel sheet.
2. Description of the Related Art
Automobile fuel tanks are generally manufactured by plating surfaces of a soft steel sheet with a lead alloy and shaping and welding the terne coated steel sheet. The continued use of lead-containing materials, however, tends to be severely limited with the increasing sensitivity to environmental issues.
Several substitutes for the terne coated steel sheet have been developed. Unfortunately, the substitutes have the following problems. Alxe2x80x94Si plating materials as lead-free plating materials are unreliable in weldability and long-term corrosion resistance and, thus, are used only in restricted fields. Although resinous materials have been tried for uses in fuel tanks, industrial use of the resinous materials which are inevitably permeable to fuel is limited under circumstances such as regulations against fuel transpiration and recycling. Also, the use of austenitic stainless steels, which requires no lining treatments, has been attempted. Although the austenitic stainless steels exhibit superior processability and higher corrosion resistance compared with the ferritic stainless steels, the austenitic stainless steels are expensive for fuel tanks and have the possibility of stress corrosion cracking (SCC). Thus, the austenitic stainless steels have not yet been used in practice.
In contrast, the ferritic stainless steels not containing nickel are advantageous in material costs compared with the austenitic stainless steels, but do not exhibit satisfactory corrosion resistance to so-called xe2x80x9cdeteriorated gasolinexe2x80x9d containing organic acids such as formic acid and acetic acid which are formed in the ambient environment. Furthermore, the ferritic stainless steels do not exhibit sufficient processability to deep drawing for forming fuel tanks having complicated shapes and to expanding and bending of the pipes for forming expanded fuel pipes and bent fuel pipes.
Japanese Unexamined Patent Publication Nos. 6-136485 and 6-158221 disclose double-layer steel sheets each including a corrosion-resistant steel layer and a low-carbon or ultra-low-carbon steel layer having excellent processability to achieve both corrosion resistance and processability. However, the double-layer steel sheets exhibit less adaptability to mass production.
The invention provides a ferritic stainless steel sheet which exhibits superior processability and high corrosion resistance to deteriorated gasoline and is useful for automobile fuel tanks and fuel pipes. In particular, the ferritic stainless steel of the invention has a thickness in the range of about 0.4 to about 1.0 mm and superior deep drawing processability, namely, an revalue of at least about 1.50 and preferably at least about 1.90.
The r-value in the invention represents a mean plastic strain ratio determined by equation (1) according to Japanese Industrial Standard (JIS) Z2254:   r  =                    r        0            +              2        ⁢                  r          45                    +              r        90              4  
wherein,
r0 is a plastic strain ratio measured using a test piece which is sampled in parallel to the rolling direction of the sheet;
r45 is a plastic strain ratio measured using a test piece which is sampled at 45xc2x0 to the rolling direction of the sheet; and
r90 is a plastic strain ratio measured using a test piece which is sampled at 90xc2x0 to the rolling direction of the sheet.
An r-value of less than about 1.50 precludes deep drawing into a complicated fuel tank shape and bending into a complicated bent pipe shape and exhibits high impact brittleness (secondary processing brittleness) even if the sheet is capable of processing.
The invention also provides a ferritic stainless steel having a surface ridging height of about 50 xcexcm or less at 25% deformation in uniaxial stretching. Ridges formed during processing of steel sheets for automobile fuel tanks are not necessarily so small because these tanks are produced by press forming of the sheet. According to our investigations, however, ridges cause cracking of the sheet during severe press forming processes which are used in the production of fuel tanks. Hence, the ridging height must be small. The ridges generated in the sheeting process vary the state of contact of the unprocessed steel sheet piece with the press die and results in xe2x80x9cgnawingxe2x80x9d or xe2x80x9cgallingxe2x80x9d due to a local deficiency of lubricant oil film. The gnawing also causes cracking along the ridges.
According to our further investigations, a steel sheet exhibiting superior press formability suitable for processing of fuel tanks having complicated shapes has a surface ridging height of about 50 xcexcm or less at a 25% deformation in uniaxial stretching. Herein, the ridges on the steel sheet generated during processing are evaluated by the height of the ridges in a direction perpendicular to the stretching direction when the steel is stretched in the rolling direction.
The invention also solves a problem in the art known in the case of severe forming of a ferritic stainless steel into fuel tanks and fuel pipes and in the case of lubricant-free press forming. That is, the invention provides a ferritic stainless steel by a lubricant-free process exhibiting superior deep drawability and requires no lubrication steps for treating the sheet with lubricant oil.
We discovered that a predetermined amount of a lubricant coat primarily containing an acrylic resin which is applied on the surfaces of a ferritic stainless steel sheet decreases the dynamic friction coefficient between the steel sheet and the press die, thus preventing xe2x80x9cgnawingxe2x80x9d and being capable of processing into articles having further complicated shapes.
We intensively investigated the effects of the composition of ferritic stainless steel sheets and the method for making the same on the corrosion resistance in deteriorated gasoline and the r-value of the ferritic stainless steel sheet and found that the corrosion resistance to the deteriorated gasoline is remarkably improved by adding appropriate amounts of Mo and V to the steel sheets.
Since the addition of Mo precludes processability, we further investigated the r-value as a reference of processability of Mo-containing steel sheets and found that a high revalue is achieved by a specified method.
Furthermore, we found that optimized annealing conditions for hot-rolled ferritic stainless steel sheets minimize the ridging height, provide superior press formability, and that the application of a lubricant coat on the steel sheet surfaces improves sliding performance in forming, decreases the dynamic friction coefficient between the steel sheet and the press die, and facilitates forming of articles having further complicated shapes.
According to an aspect of the invention, a ferritic stainless steel sheet for fuel tanks and fuel pipes comprises, by mass percent, about 0.1% or less of C; about 1.0% or less of Si; about 1.5% or less of Mn; about 0.06% or less of P; about 0.03% or less of S; about 1.0% or less of Al; about 11% to about 20% Cr; about 2.0% or less of Ni; about 0.5% to about 3.0% Mo; about 0.02% to about 1.0% V; about 0.04% or less of N; at least one of about 0.01% to about 0.8% Nb and about 0.01% to about 1.0% Ti; and the balance being Fe and incidental impurities.
Preferably, the ferritic stainless steel sheet has a ridging height of about 50 xcexcm or less at a 25% deformation in uniaxial stretching.
Preferably, a lubricant coat comprising an acrylic resin, calcium stearate, and polyethylene wax is coated by baking on the surfaces of the ferritic stainless steel sheet in a coating amount of about 0.5 g/m2 to 4.0 g/m2.
According to another aspect of the invention, a method for making a ferritic stainless steel sheet for fuel tanks and fuel pipes, comprises the steps of rough-rolling a slab comprising, by mass percent, about 0. 1% or less of C, about 1.0% or less of Si, about 1.5% or less of Mn, about 0.06% or less of P, about 0.03% or less of S, about 1.0% or less of Al, about 11% to about 20% Cr, about 2.0% or less of Ni, about 0.5% to about 3.0% Mo, about 0.02% to about 1.0% V, about 0.04% or less of N, at least one of about 0.01% to about 0.8% Nb and about 0.01% to about 1.0% Ti, and the balance being Fe and incidental impurities; hot-rolling the rough-rolled sheet under a linear pressure of at least about 3.5 MN/m at a final pass in the finish rolling; cold-rolling the hot-rolled sheet at a gross reduction rate of at least about 75%, the cold-rolling step including one rolling stage or at least two rolling stages including intermediate annealing; and annealing the cold-rolled sheet.
Preferably, the hot-rolled sheet is subjected to hot-rolled sheet annealing according to the following equations:
900xe2x89xa6T+20txe2x89xa61,150and txe2x89xa610
wherein T is the annealing temperature (xc2x0 C.) and t is the holding time (minutes).
Preferably, a lubricant coat comprising an acrylic resin, calcium stearate, and polyethylene wax is coated by baking on the surfaces of the hot-rolled or annealed hot-rolled sheet in a coating amount of about 0.5 g/m2 to about 4.0 g/m2.